Thanks. I am not very good in codes. If I specify my conditions, would you be able to you help me? Each post has only one category, so no multiple categories. For example: I have category Good Jokes (ID 19) and I want example.com/19.jpg image for this category. I have about 10 categories and I want specific image for each. What should I edit / add to code?
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Martin HorvathDec 9 '12 at 16:11

Use FPW Category Thumbnails plugin. It does exactly what you need and no coding is required. Map images to their respective categories. If you have many posts already use Apply Mapping button and all posts belonging to the category which has an image mapped to it will get this image as a thumbnail. From this point on everything is automated. When you create or edit a post it will get its thumbnail according to the plugin's mapping on save. As an author of this plugin I'm sure you will not be disappointed!